1. Field of the Invention
This invention relates to the recovery of non-ferrous metals, particularly aluminum, from dross resulting from secondary melting and remelt operations.
2. Discussion of the Prior Art
Dross is a material which forms on the surface of molten non-ferrous metals during remelting and metal holding and handling operations when the molten metal is in contact with a reactive atmosphere, such as air. Dross normally consists of metal oxides entraining a considerable quantity of molten free (un-reacted) metal, and for economic reasons it is desirable to extract the free metal before discarding the residue.
The traditional processes for recovering the free metal generally involve one or several of the following steps:
1. Cooling of the dross using either a mechanical cooler or a dross room (where the dross is spread out over a floor). PA1 2. Transportation of the cooled dross to a dross treatment plant. PA1 3. Crushing and screening of the cooled dross, followed by elimination (usually by dumping) of the fine fraction which consists mainly of oxides and which may be dangerous and disadvantageous to handle in subsequent steps. PA1 4. Heating of the larger dross fractions in the presence of a molten salt bath in order to remelt the metallic fraction and cause the resulting molten droplets to coalesce.
Step 4 is normally carried out in a rotary furnace. The dross fractions and salt mixture are charged and heated to a temperature above the melting point of the metal using direct flame burners. The furnace is then rotated at a suitable rate of speed to obtain a tumbling or cascading action of the mixture. Examples of such processes are disclosed for example in U.S. Pat. No. 3,676,105 to McLeod et al, U.S. Pat. No. 3,789,024 to Murphy et al, and U.S. Pat. No. 4,030,914 to Papafingos et al, all of which disclose the use of salt mixtures as fluxes.
All salt mixtures normally used in Step 4 are selected to be molten at the operating temperatures in order to protect the molten metal surface, and sufficient quantities must be added to completely cover the molten metal. The salt mixtures most frequently used are chloride-based in order to have a sufficiently low melting point, but frequently have added fluoride salts, such as cryolite. The fluoride salts are added for various reasons including improving the fluidity and wettability of the molten salt, and improving the solubility of oxides in the molten salt mixture. A typical salt mixture consists of a 50:50 mixture of NaCl and KCl to which a small quantity (for example 5 wt. %) of a fluoride salt, such as cryolite, is added.
Disadvantageously, the molten salts generate salt fumes which are corrosive both within the plant and in the external environment, and the residual salt cake containing dross impurities is very polluting because the salts are water soluble and are readily leached out of dump sites.
Processes have been proposed which use some form of heating by direct plasma means (Dube et al, U.S. Pat. No. 4,952,237; Lindsay, U.S. Pat. No. 4,997,476; Drouet, U.S. Pat. No. 5,245,627) or by an indirect electrical method (Montanga, U.S. Pat. No. 3,999,980) to permit heating of the dross to temperatures sufficient to melt and release the molten metal without using molten salt fluxes to protect the furnace charge. These methods work by controlling the furnace atmosphere to protect the charge from reaction with excessive amounts of oxidizing gases (oxygen, water, carbon dioxide) during processing, thus making it possible obtain acceptable recoveries. However, these methods do not allow all of the metal trapped in the dross oxide layers to be completely released by heating alone and the recoveries, particularly from drosses generated in secondary melting and remelting processes, are lower than desirable. Recoveries may be as much as 6% less in these processes than processes based on conventional molten salt treatment methods and the resulting economic penalty can be severe.